Acetabular cup prosthesis and method of forming the prosthesis

ABSTRACT

An acetabular cup prosthesis comprising an acetabular cup having a rim and comprising a metal band applied around the outer circumference of the acetabular cup prosthesis and adjacent to said rim.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/572,039, filed Oct. 1, 2009, which claims priority to GB PatentApplication No. 0818326.1, filed Oct. 7, 2008 and GB Patent ApplicationNo. 0818505.0, filed Oct. 9, 2008. Each of these applications is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a prosthesis. More particularly, itrelates to a preassembled acetabular component for a hip prosthesis anda process for the production thereof.

BACKGROUND OF THE INVENTION

The efficient functioning of the hip joint is extremely important to thewell-being and mobility of the human body. Each hip joint is comprisedby the upper portion of the femur which terminates in an offset bonyneck surmounted by a ball-headed portion which rotates within theacetabulum in the pelvis. Diseases such as rheumatoid- andosteo-arthritis can cause erosion of the cartilage lining of theacetabulum so that the ball of the femur and the hip bone rub togethercausing pain and further erosion. Bone erosion may cause the bonesthemselves to attempt to compensate for the erosion which may result inthe bone becoming misshapen.

Operations to replace the hip joint with an artificial implant arewell-known and widely practiced. Generally, the hip prosthesis will beformed of two components, namely: an acetabular component which linesthe acetabulum; and a femoral component which replaces the femoral head.The femoral component may be total femoral head replacement in whichcase the component includes a head, neck and a stem which in use isinserted into the end of a prepared femur. Alternatively, whereappropriate, the femoral head component may be a resurfacing prosthesiswhich is attached to the head of the femur once it has been suitablymachined.

In an operation to insert a prosthetic acetabulum in a patient's pelvisthe surgeon first uses a reamer to cut a cavity of appropriate size inthe patient's pelvis. An acetabular cup is then inserted into thecavity. By “appropriate size” is meant a size which is selected by thesurgeon as being the most appropriate for that particular patient.Normally, it is desirable to retain as much of the original healthy bonesurface as possible.

Commercially available acetabular cups are sold in a range of sizes tosuit the needs of individual patients. Generally, acetabular cups areavailable in sizes of from 42 mm to 62 mm diameter with 2 mm incrementsbetween neighboring sizes.

There are a number of different types of prosthetic acetabular cups. Onetype of cup is those made from polyethylene. They are generally cementedinto the acetabulum and require only light pressure to seat them in thecement.

One alternative cup type has a polyethylene liner unit for articulationwith the femur and a metal shell for insertion into the pelvic cavity.These cups with metal shells may be implanted without cement such thatthey rely on a jam fit between the metal shell and the patient'sacetabulum. However, in some arrangements, screws may be used to securethe cup shell in position in the pelvis before the liner is applied intoposition. The insertion of the metal shell into the pelvis requiresconsiderable force. As the surgeon applies this force, there is a riskthat the metal shell can become damaged or deformed. There is also apossibility that during the application of the force, the shell may bemoved so that it is not in the optimum alignment in the acetabulum.Often the metal shells have outer surfaces or coatings which encouragebone to grow into them over time.

With this type of prosthesis, the polyethylene liner unit is snapped orscrewed into the metal shell after the metal shell has been seated inthe acetabulum. Thus the inner surface of the liner forms the socketpart of the joint.

More recently, ceramics have been used to as an alternative to theplastics liner. In this arrangement, the metal shell, which is generallyformed from titanium and which is of a similar thickness to thearrangement in which a polyethylene liner is used, is inserted into theacetabulum. The ceramic liner is then inserted into the shell. It can bedifficult for the liner to be accurately aligned in the shell. Inaddition, this insertion of the liner does require the application of aconsiderable force which is usually applied by the surgeon using amallet often via an insertion tool. Considerable force is generallyrequired to achieve a successful interface. However, this force candamage the ceramic liner.

In order to get an optimum fit, it is necessary that the forces appliedfor both the insertion of the metal shell and for the ceramic liner areappropriate but not excessive. One problem however, is that to datethere has been no understanding as to what forces are appropriate nor isthere a means to ensure that the correct force is applied.

The surgeon is not generally able to apply a controlled amount of forceapplied. Some surgeons may not apply sufficient force in one hit and itmay be necessary for a plurality of hits to be used. These may not allstrike at the same angle and may not each apply the same force. Othersurgeons may apply a much greater single strike. The force applied bythe surgeon on, for example, an insertion tool may vary considerably andcan be of the order of about 3 to 5 kN but can also be much higher andmay even be of the order of about 35 kN.

Whilst very large forces may only be applied for small moments in time,of the order of seconds or fractions of a second, forces of thismagnitude, or a plurality of forces of smaller magnitude may cause theshell to be deformed as it is inserted into the acetabulum. This is aparticular risk in those arrangements where the thickness of the shellis only from about 1 mm to about 3 mm thick. If the shell is deformed,it can become difficult or even impossible to insert the liner.

Additionally or alternatively, the liner may be incorrectly seated inthe shell which can lead to various disadvantages. Not only is there arisk that where a portion of the liner stands above the rim of the cup,a point of irritation can be produced but also, there is a risk thatmaterial, such as wear debris, may congregate against the raised portionof the liner or against the wall of the cup in the area where the linersits below the rim. This accumulation of debris may provide a site forpost-operative infection. Even if the liner is correctly located and theshell is not deformed during the assembly process, it may becomedeformed on insertion of the prosthesis into the pelvis such that theshell may become spaced from the liner over at least a portion of theprosthesis.

Even if the surgeon is able to accurately seat the liner in the cup,there is a risk that during assembly debris may be caught between theliner and the cup which may effect the wear properties of theprosthesis. A further problem associated with the presence of debris,which may include fluids such as blood or fat, between the shell andliner is that in use, in vivo the presence of the debris may cause theshell and liner to move apart.

Without wishing to be bound by any particular theory, it will beunderstood where the shell and ceramic liner are held together byfriction, debris, in particular fatty substances or blood, can interferewith the frictional interface between the outer surface of the liner andthe inner surface of the shell such that there is a propensity for theliner to move out of the shell.

A further problem which may be encountered is that while inserting theliner in the shell it may become damaged. If this damage is a chip orcrack on the outer surface of the liner, i.e., on the surface adjacentto the surface of the shell, its presence may not be noticed by thesurgeon during assembly. However, its existence will be a point ofweakness which can result in the prosthesis failing in use.

One solution that has been proposed is to provide a preassembled unitacetabular cup prosthesis comprising: an outer shell; and a ceramicliner located within the shell. In one arrangement, the preassembledunit may be assembled ex-vivo under a controlled force selected tooptimise the pre-stressing of the components of the prosthesis.

This arrangement provides acetabular components which reduces the riskof liner misplacement and which has enhanced life expectancy arising, inpart, through improved resistance to damage caused during impaction intothe acetabulum. It is also desirable to provide an acetabular cupprosthesis which can be easily handled and inserted during surgerywithout damage to the acetabular cup prosthesis and which minimizes therisk of debris being trapped between the cup and the liner.

Examples of such preassembled acetabular cup can be found in copendingEuropean applications 08103811.9 and 08103809.3 filed on 2 May 2008.Whilst the arrangements describe various arrangements which address theaforementioned problems, there is a need for alternative arrangements.

In certain arrangements it may be desirable to provide an acetabular cupprosthesis which is made substantially from ceramic.

Whilst acetabular cup prosthesis made from ceramic offer variousadvantages, they may suffer from various disadvantages. The ceramic maynot have sufficient strength to withstand the rigors of insertion.Further even if the ceramic cup can be successfully inserted in thepelvis without damage, due to the structure of the surface there may belittle or no torsional stability.

Even if the acetabular cup does include a thin metal shell, there may bea need to provide additional strength or additional torsional stability.

SUMMARY OF THE INVENTION

Thus according to the present invention there is provided an acetabularcup prosthesis comprising an acetabular cup having a rim and comprisinga metal band applied around the outer circumference of the acetabularcup prosthesis and adjacent to said rim.

The presence of the metal band applied around the outer circumference ofthe acetabular cup prosthesis and adjacent the rim thereof providesadditional strength to the prosthesis and in particular provides therequired level of hoop compression.

The acetabular cup of the present invention may be formed of anysuitable material. In a preferred embodiment the suitable material willbe a ceramic. For the purposes of this application, the term “ceramic”should be construed as meaning not only true ceramic materials but alsoother materials which display ceramic-like properties. Ceramic-likeproperties for the purposes of the present invention are those wherestrength, stiffness and rigidity are similar to those of ceramics.Examples of suitable materials include glasses. In one arrangement ametal shell may be provided.

The prosthesis of the present invention provides a prosthesis withimproved properties over prior art prosthesis. In particular, theprosthesis of the present invention has improved resistance to fracture.In particular where the cup is a ceramic cup or includes a ceramicliner, the presence of the band will serve to pre-stress the ceramic.This pre-stressing occurs during the controlled assembly process.Unstressed ceramic liners such as those of the prior art, are prone tofracture in use. Stressing of the components of the prosthesis isdiscussed in more detail below.

The band of the prosthesis is preferably made from metal. Any suitablemetal may be used, with titanium being particularly preferred.Cobalt/chromium may also be used. The outer surface of the band may beprovided with one or more ribs extending away from the cup. In use theseribs will interact with the pelvis to provide the required torsionalstability. In one arrangement one or more, preferably three, ribs may belocated at a plurality of points around the circumference of the band.In one arrangement one or more, preferably three ribs, may be located atthree equally spaced positions around the circumference.

The ribs may be of any suitable configuration. Where there is one ormore ribs present, the ribs may be of the same or of differentconfigurations. In one arrangement, the or each rib may be alongitudinal extension.

In one arrangement, the band may be configured on its out surface topromote bone integration. In one arrangement, the outer surface may becoated with a bone growth promoting material such as hydroxyapatite.

In the embodiment where a shell is used, this may be coated on its outersurface with material to promote bone integration. In one arrangement,the outer surface may be coated with a bone growth promoting materialsuch as hydroxyapatite.

Although not preferred, where a shell is used it may be a titaniumshell. If used, the titanium shell has a thickness in the region ofabout 1 mm to about 3 mm.

The ceramic cup or liner may be formed of any material which hasacceptable biocompatibility, hardness and wear resistance. Suitableceramic materials include silicon nitride, doped silicon nitride, analumina-zirconia ceramic, yttria, stabilized zirconia, ceria, stabilizedzirconia, zirconia ceramics, alumina ceramics, oxinium or mixturesthereof. The thickness of the ceramic cup or liner is preferably in theregion of from about 2 mm to about 5 mm.

Where the band is applied directly to the outer surface of a ceramiccup, the ceramic will generally be shaped in the region of the rim wherethe band is applied such that when the band is in position, the externalprofile of the prosthesis (ignoring any ribs or other bitingconfigurations) will correspond to that of a ceramic cup prosthesis nothaving a band. Thus the cup with the band will still have theapproximately hemispherical outer configuration.

It will therefore by understood that the thickness of the band willgenerally be of the order of a few millimeters. The surface of the bandwhich will be in contact with the cup will generally be flat whereas theouter surface of the band may be curved and as such the shape of theband in cross-section may be D-shaped. At the thickest point it may havea thickness of the order of about 1 mm to about 3 mm.

The band may be applied to the acetabular cup by any suitable means. Inone arrangement, it may be press fitted onto the cup. Since the band isa tight fit on the cup press fitting onto the cup may presentdifficulties. In one arrangement, the band may be heated to allow it tobe correctly fitted. In one arrangement, the heating may be by inductionheating. A benefit of induction heating is that only the metal band willbe effected and the ceramic cup will not be heated. The inductionheating may be carried out by any suitable means. The press fit mayrequire load to be applied. The load required may depend on thetemperature to which the band is heated. In one arrangement, heating maybe to from about 500 to about 700 C. In one arrangement approximately 5kN load may be used for the press-fitting.

Thus according to a second aspect of the present invention there isprovided a process for forming the acetabular cup prosthesis of theabove first aspect comprising the step of heating the band beforeapplying it to the cup.

Once the band has cooled, the loading of the band on the cup willgenerally act to pre-stress the cup in an optimum manner such thatseparation between the band and the cup will not occur. In thisconnection it will be understood that the band being metal is strong inhoop tension whereas the ceramic cup is strong in hoop compression. Withthe pre-stressing of the present invention, the tensions are optimised.In particular, the residual stress in the ceramic may be engineered to avalue that optimises performance and is compressive. In addition, thecompressive interface between the two components is maintainedthroughout the entirety of the loading of the prosthesis in vivo.

A further advantage of the present invention is that the acetabular cupprosthesis of the present invention has sufficient strength to withstandthe forces supplied during the insertion of the prosthesis into theacetabulum without damage, distortion or separation of the band from thecup. In particular, and surprisingly, the sphericity of the prosthesisis substantially maintained even though the diameter is reduced due tothe localised compression at the rim of the liner.

The prosthesis of the present invention may be provided with animpaction cap. The impaction cap may be pre-assembled with theprosthesis as described in EP2008618 which is incorporated herein byreference. In one arrangement the impaction cup is configured such thatthe force applied to impact the prosthesis into the acetabulum isdirected via the ceramic cup so that any shock passing through the metalband is minimised.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 is a perspective view of the acetabular cup prosthesis of thepresent invention;

FIG. 2 is a schematic diagram of a cross section through a cup preparedto accept the band; and

FIG. 3 is a schematic diagram of a cross section through a portion of anacetabular cup prosthesis of the present invention including the band.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the prosthesis 1 of the present inventioncomprises a cup 2 and a titanium band 3 located around the externalsurface of the cup and adjacent to the rim 4 of the cup 2. Ribs 5 arelocated on the external surface of the band and are configured such thatwhen located in the pelvis torsional movement of the cup in the pelvisis resisted.

The shape of the cup 2 before the band 3 is applied is illustratedschematically in FIG. 2. As illustrated in FIG. 3, the band 3 is shapedso that the hemispherical external surface of the cup is completed.

What is claimed is:
 1. An acetabular cup prosthesis comprising anacetabular cup having a rim and comprising a metal band applied aroundthe outer circumference of the acetabular cup prosthesis and adjacent tosaid rim wherein the outer surface of the metal band has a plurality ofelongated ribs extending along a width of the metal band and projectingaway from the cup, the ribs being circumferentially spaced from eachother about the metal band, the metal band in cross-section beingD-shaped.
 2. An acetabular cup according to claim 1 wherein theacetabular cup is formed from ceramic.
 3. An acetabular cup according toclaim 2 wherein the metal band of the prosthesis is made from titanium.4. An acetabular cup according to claim 1 wherein the metal band of theprosthesis is made from titanium.
 5. An acetabular cup according toclaim 1 wherein the metal band at the thickest point has a thicknessfrom about 1 mm to about 3 mm.
 6. A method of forming the acetabular cupprosthesis of claim 1 wherein the metal band is press fitted onto thecup.
 7. A method according to claim 6 wherein the metal band is heatedprior to or during press fitting.
 8. A method according to claim 7wherein the heating is achieved by induction heating.